Petroleum naphtha stocks, that is, petroleum naphthas of C.sub.5 -400.degree. F. (about C.sub.5 -200.degree. C.) boiling range may be converted to high octane gasoline by catalytic reforming. The low boiling (C.sub.5 to C.sub.7) paraffinic hydrocarbons, referred to as light straight run (LSR) naphthas which comprise a significant proportion of the full range of naphtha are, however, difficult to reform and for this reason, are usually removed from the full range naphtha by fractionation in order to improve reformer performance. Although the LSR naphtha generally has a boiling point in the gasoline range, for example, from 35.degree. to 90.degree. C. (94.degree. to 193.degree. F.) they are generally deficient in terms of octane quality because of their relatively high content of straight chain, n-paraffins. In spite of this defect, however, it has become common practice to include these napthas in the gasoline pool and to make up the octane deficiency by the addition of high octane reformate and lead-containing octane improvers. Because the use of lead octane improvers must now be reduced and possibly eliminated in the near future, there is currently a need for supplementing the octane rating of the gasoline pool by other methods. Although the amount of reformate in the pool could be increased, reforming is relatively expensive and because reforming capacity in a refinery may be limited, it is often desirable to seek other alternatives for improving octane. Furthermore, the inability to use lead-containing octane improvers in the gasoline pool coupled with the limitations on reforming capacity will mean that large amounts of light straight run (LSR) naphtha will have to be diverted from the gasoline pool to other possible uses, e.g. as olefin feed stock, although this has the disadvantage that the final products may be of relatively low value compared to the gasoline which previously used the LSR as such (Oil and Gas Journal, June 3, 1985, p. 47). Even if reforming capacity were available to deal with the LSR, the further problem which will be encountered is that LSR naphtha has extremely poor reforming characteristics because of its paraffinic nature so that even if reforming capacity were available, it will be largely misused if LSR were to be used as the feedstock. Accordingly, it would be desirable to devise some method for upgrading the full range naphthas which are available in refineries to form gasoline products of improved octane characteristics and, in addition, to ensure that the LSR portion of these naphthas has a sufficiently high octane rating to permit it to be blended directly into the gasoline pool without the need for lead-containing octane improvers.
Various proposals have been made in the past for improving the octane performance of various naphthas and these have generally used zeolite catalysts such as HY or HZSM-5 as described, for example, in U.S. Pat. Nos. 4,191,634 and 4,304,657. Alternatively, they have used catalysts similar to reforming catalysts containing chlorided noble metals such as chlorided-platinum-alumina-rhenium, as described in U.S. Pat. No. 4,241,231. Many of these catalysts systems have been undesirable from various points of view. For example, the chlorided reforming type catalysts require regeneration and rejuvenation and because the process is endothermic in nature, it requires the use of high temperatures, e.g. 350.degree. to 420.degree. C. with a constant high heat input. Systems using relatively small pore size zeolites, for example, the intermediate pore size zeolites such as ZSM-5 have other disadvantages, including excessive conversion of the naphtha to C.sub.3 and lighter products. Thus, there is a continuing need for refinery processes which are capable of converting naphthas to high octane gasoline in good yields.